Japanese encephalitis (JE)

Japanese Encephalitis (JE) is a viral disease that affects the central nervous system and is caused by vector-borne Japanese encephalitis virus (JEV). JE is primarily a disease of children, and most adults in endemic countries would have acquired natural immunity after childhood infection. However, all age groups could be potentially infected. The incubation period of JE ranges between 4-14 days. Most JEV infections are mild (fever and headache) or without apparent symptoms, but approximately 0.4% of infections result in severe clinical illness. According to a 2015 WHO report, there are approximately 68,000 clinical cases of JE annually. Acute clinical JE disease is characterized by high fever, headache, stiff neck, abnormal movement, coma and spastic paralysis. The fatality rate ranges between 20–30% among reported infections. Permanent intellectual, neurologic or psychiatric sequelae can occur in 30–50% of those who survive encephalitis.

JE transmission is primarily via zoonotic cycles involving mosquitoes (culicine mosquito) and several vertebrate species. While domestic pigs and wild birds serve as reservoirs and amplifying hosts, JEV is primarily transmitted to humans through bites from infected mosquitoes. Due to the vector-borne feature, the risk of JE transmission varies by season and geographic location. In temperate areas, transmission primarily occurs in summer and autumn, when the number of active mosquitoes is abundant. The risk of transmission is higher in rural areas, especially those in Southeast Asia and Western Pacific regions where pigs are raised and where agricultural practices are prevalent.

Vaccination is the most effective prevention strategy for JE. There are four vaccines available. The live attenuated JE vaccine made from the Beijing strain is the most widely used due to its ability to induce stronger and broader neutralising antibodies. A single dose gives more than 96% long-lasting immunity, which is similar to that induced by natural infection. There is no antiviral cure for patients with JE, but the available treatment can relieve symptoms and stabilise the patient. Interventions, such as environmental, vector and amplifying hosts control have been undertaken in high risk countries. However, there is little evidence to support that these mechanisms are effective in reducing disease burden.

The vaccination strategy for JE combines both routine vaccination and supplementary immunisation activities (SIA). One-time large-scale catch-up campaigns targeting all children between 1 – 15 years old are funded by Gavi, the Vaccine Alliance. Along with Gavi and/or non-Gavi funded immunisation, WHO recommends strengthening surveillance and reporting mechanisms to further control and prevent the disease.